The present invention relates to a method and device for stretching and folding a strut type jib for a wheel type crane such as a truck crane and a rough terrain crane, wherein the jib is stretched from a forward end of a boom in an operative condition of the jib, while the jib is being folded along the boom in an inoperative condition of the jib.
Conventionally, the following jib stretching and folding systems are known.
(1) In a jib twisting system as disclosed in Japanese Patent Publication No. 63-3827, a strut type jib (a jib consisting of right and left beams and a plurality of connecting bars connecting the right and left beams and having a flat tapering ladder-like structure) is folded along one side surface of a boom under a vertically disposed condition of the jib where opposite flat planes of the jib are oriented horizontally, and a base end portion of the jib is directed to a forward end portion of the boom. In an operative condition of the jib, a lower end portion of the base end portion of the jib is connected to the forward end portion of the boom, and the jib is suspended from this connection point as a pivotal point. Then, the jib is twisted at an angle of about 270 degrees, and is then swung up to be stretched forwardly from the forward end of the boom.
(2) In a jib swinging system as disclosed in Japanese Patent Publication No. 63-30278, a jib folded along one side surface of a boom under the vertically disposed condition of the jib is turned to be positioned under a lower surface of the boom. Under this condition, a base end portion of the jib is then connected to a forward end portion of the boom. Then, the jib is suspended from this connection point as a pivotal point. Thereafter, the jib is swung up to be stretched forwardly from the forward end of the boom.
(3) In a jib sliding system as disclosed in Japanese Patent Laid-open Publication No. 55-135094 and Japanese Utility Model Laid-open Publication No. 62-147687, a jib folded along a lower surface of a boom is slid forwardly, and a base end portion of the jib is connected to a forward end portion of the boom. Under this condition, the jib is stretched forwardly from the forward end of the boom.
However, all the above conventional systems have the following defects.
As both the jib twisting system mentioned in Paragraph (1) and the jib swinging system mentioned in Paragraph (2) include a jib suspending step, the jib must be swung up from the jib suspended condition. Accordingly, in the case that an obstacle such as an electric cable exists over the jib, the stretching and folding operation of the jib cannot be carried out.
In contrast, the jib sliding system mentioned in Paragraph (3) has an advantage such that the jib can be stretched and folded under the horizontal condition of the boom. However, since the jib is folded along the lower surface of the boom, the jib hinders a field of side view in the running of a crane, especially a rough terrain crane in which an operator cab is used commonly in both operating the crane and running the crane, and the boom extends to one side of the operator cab in running the crane.